Traditional oscillation circuits can by categorized into RC oscillators, LC oscillators and crystal oscillators. The major component designed in the basic structure of these circuits is a calculation magnifier. However, the oscillation circuits based on the design of a calculation magnifier is constrained by the gain bandwidth product, which is a big deficiency for the design of high-frequency circuits, for example, the Wien-bridge oscillator. As for the LC oscillators, such as Hartley and Colpitts oscillators, the components therein are easily affected by either the temperature coefficients or the stray capacitances/resistances thereof and resulted in change of the oscillation frequency. Usually, the temperature coefficients are higher than 100 ppm/° C. (which equivalent to a change of 0.03% per 10° C.). Therefore, the stability of the LC oscillators is not good.
Instead of the traditional oscillators, frequency synthesizers can be utilized for generating RF signals have frequencies of tens of Giga Hertz, while suffering extensive power consumption and phase noises. Another method is to input a low-frequency signal to a frequency multiplier, to generate high oscillation frequencies. However, the method has deficiencies such as low output power and poor harmonic rejection ratio, HRR.
Please refer to FIG. 1, which schematics the signal generates by a push-push oscillator known to the art. According to FIG. 1, a first fundamental frequency signal S1 having a first fundamental frequency f01 is a sine wave having a positive phase. A second fundamental frequency signal S2 having a second fundamental frequency f02 is a sine wave having a negative phase. The first fundamental frequency f01 is equal to the second fundamental frequency f02. The second-order harmonic of the first fundamental frequency signal S1 has a frequency which is two times of the first fundamental frequency f01. The second-order harmonic of the second fundamental frequency signal S2 also has a frequency which is two times of the first fundamental frequency f01. The second-order harmonics of the first and the second fundamental frequency signals, S1 and S2, have the same phase. Therefore, the amplitudes of the two second-order harmonics can be added and a double-frequency signal S3 is obtained. The double-frequency signal S3 has a double fundamental frequency 2 f01, which is two times as much as the first fundamental frequency f0l. The phases of odd-number-order harmonics of the first fundamental frequency signal S1 are opposite to that of the second fundamental frequency signal S2 and therefore each couple of the odd-number-order harmonics eliminate each other.
Please refer to FIG. 2(A), which is a circuit diagram of the push-push oscillator known to the art. The push-push oscillator 20 comprises transistors 201 and 202, inductors 203 and 204, a current source 205 and a conducting line 206. The inductor 203 has an end A and an end B. The inductor 204 has an end C and an end D.
According to FIG. 2(A), the gate G10 of the transistor 201 is coupled to the drain D20 of the transistor 202, the gate G20 of the transistor 202 is coupled to the drain D10 of the transistor 201, the source S10 of the transistor 201 and the source S20 of the transistor 202 is coupled to the current source 205, the end A of the inductor 203 is coupled to the drain D10 of the transistor 201, the end B of the transistor 203 is coupled to the conducting line 206, the end C of the inductor 204 is coupled to the drain D20 of the transistor 202, and the end D of the inductor 204 is coupled to the conducting line 206.
The push-push oscillator 20 generates the first fundamental frequency signal S1 at the drain D10 of the transistor 201, and the second fundamental frequency signal S2 at the drain D20 of the transistor 202. The double-frequency signal S3 having the double fundamental frequency 2f01 is obtained at the junction of the conducting line 206, end B and end D.
Please refer to FIG. 2(B), which is a circuit diagram of the injection-locked frequency multiplier known to the art. The injection-locked oscillator 21 comprises the push-push oscillator 20 excepting the conducting line 206, and further comprises transistors 211 and 212, buffers 213 and 214, and a current source 215. The buffers 213 and 214 have an input and an output terminals, in+ and out+, and in− and out−, respectively.
The circuit layout is illustrated in FIG. 2(B). According to FIG. 2(B), a differential signal SD1 having a fundamental frequency f is inputted at the gate G30 of the transistor 211 and the gate 40 of the transistor 212 respectively. The differential signal SD1 includes a DC and an AC components. The transistors 211 and 212 are biased at non-linear regions by the DC component. Passing through the transistor 211, the AC component is transformed and then generates a harmonic signal SD2, which includes a positive-phase triple-frequency harmonic component having a triple fundamental frequency 3f1, at the drain D30. The positive-phase triple fundamental frequency 3f1 is three times of the fundamental frequency f. The harmonic signal SD2 is inputted at the input terminal in+ of the buffer 213. Similarly, passing through the transistor 212, the AC component is transformed and then generates a harmonic signal SD3, which includes a negative-phase triple-frequency harmonic component having a triple fundamental frequency 3f2, at the drain D40. The negative-phase triple fundamental frequency 3f2 is three times of the fundamental frequency f. The harmonic signal SD2 is inputted at the input terminal in− of the buffer 214.
Although the injection-locked frequency multiplier 21 can inject as well as lock the oscillation frequency of the signal from the push-push oscillator 20, the oscillation frequency of the signals generated by the injection-locked frequency multiplier 21 are easy to be disturbed by other harmonics. The method of injecting all the harmonics into the push-push oscillator 20 while suppressing unwanted frequencies by employing the mechanism of injection-locking thereof may result in poor harmonic rejection ratio, which affects the quality of the oscillation signals.